1. Field of the Invention
The present invention relates to methods and devices for increasing the accuracy of a digital phase modulator. In particular, the invention pertains to phase modulators of fiber optic signal signal transmitting or measuring devices, such as rate of rotation sensors (fiber optic gyroscope) that include binary-weighted planar electrodes in parallel.
2. Description of the Prior Art
Digital phase modulators, such as those described as a component of an integrated optical chip for a fiber optic gyroscope in U.S. Pat. Nos. 5,137,359 and 5,400,142 include very technically complex, precise, having low-noise high-speed digital-to-analog converters having very linear driver amplifiers. The digital signal resulting from a preprogrammed algorithm (e.g. inside an automatic control system in the case of a closed loop fiber-optic gyro) can then be fed fed directly directly to the binary-weighted electrodes of the phase modulator.
The manufacture of high accuracy phase modulators with a resolution of more than 8 bits is complex and expensive. Unfortunately, a resolution of at least 10 to 12 bits is required for high precision fiberoptic gyros of approximately 1xc2x0/h stability. This imposes extremely high manufacturing tolerance requirements, as well as heat and aging resistance on the integrated optical chip.
It is therefore the object of the present invention to reduce the stringent production and operational requirements placed on digital phase modulators without any sacrifice of accuracy.
The present invention addresses the preceding and other objects by providing, in a first aspect, a method for increasing the accuracy of a digital phase modulator of a fiber optic signal transmitting of measuring device. Such phase modulator is of the type that includes binary-weighted electrodes. The method includes storing correction values that are assigned individually to the electrodes to the phase modulator in a memory table. Thereafter, an electrode drive signal is corrected upon activation of the phase modulator in accordance with the stored correction values determined for the electrode.
In another aspect, the invention provides a method for increasing the resolution of a digital phase modulator in a fiber optic signal transmitting or measuring device wherein the modulator includes n binary-weighted electrodes. A less significant component m of a drive signal, generated in the signal transmitting or measuring device, comprising (n+m) bits for the phase modulator, is subjected to digital-to-analog conversion. Thereafter, the resultant analog signal is input to a further separate electrode of the phase modulator.
In a third aspect, the invention comprises apparatus for increasing the accuracy of a digital phase modulator of a fiber optic signal transmitting or measuring device, the modulator being of the type that includes binary-weighted electrodes. Such apparatus includes a digital control unit for providing a digital electrode drive signal corresponding to a desired modulation value at the phase modulator. A tabular memory is provided for containing correction values to be assigned to individual electrodes of the phase modulator. A switching device is provided through which the correction values can be switched as a linearized, corrected digital excitation signal to the electrodes to be activated in a fashion typical of being activated by the control unit in accordance with a respective electrode drive signal and logically combined with the electrode drive signal.
The preceding and other features of this invention will become further apparent from the detailed description that follows. Such description is accompanied by a set of drawing figures. Numerals of the drawing figures, corresponding to those of the written description, point to the features of the invention with like numerals referring to like features throughout.